Plastic parisons or preforms are conventionally reheated and formed into hollow plastic articles by blow molding in a blow mold using high pressure fluid, with if desired using a stretch rod for stretch blow molding.
The preforms are preheated in ovens immediately prior to being transferred into a blow mold. Stretch rods may be incorporated with the blow mold and typically enter the preforms from its open end and extend during the blow stage to cause biaxial orientation of the blown article.
A two stage reheat blowing machine of the type described in U.S. Pat. Nos. 4,793,960, 4,690,633 and 4,522,581 uses a series of pallets to carry preforms through a series of processing stages: thermal conditioning, stretch-blow molding and strip-off or ejection. At the stretch-blow molding station it is important that the preform is accurately aligned with the blow mold to insure that the finished blown article is symmetrically formed with respect to the neck finish. It is also important that the means used to seal the interior of the preform effectively prevent the escape of the high pressure, typically up to 700 psi, fluid used to blow the preform within the closed mold. Loss of the fluid reduces the effectiveness of the blowing, creates noise, and may cause the neck finish to be deformed if leakage occurs in that area.
Prior art machines for stretch blowing and using preforms mounted on traveling pallets by means of mandrels include U.S. Pat. Nos. 4,505,664, 4,426,202, 4,470,797, 4,487,568 and 4,456,447. Usually the mandrel is fitted inside the preform's neck finish with a minimum gap to reduce air leakage. The '568 and '447 patents show external latches to hold the preform on the mandrel and the '797 patent shows a spring loaded detent for the same purpose. In these cases, the preform is depending from the mandrel/holder and unless restrained would possibly fall off, given the amount of machine vibration and motion that it is subjected to.
Prior art machines simply close the blow mold halves trapping the preform's neck finish between the mold halves and the inner mandrel, thus initially achieving an air tight seal. However, the high pressure blowing fluid may cause the neck finish to separate from the mandrel and allow the fluid to leak, and also act on the inner wall of the neck finish causing it to deform. This deficiency is particularly prone to occur when large diameter preforms are being blown. Here the large projected area of the neck finish can be easily deformed when exposed to the blowing fluid. Prior art machines rely on the engagement of the mold directly with the preform to align the preform within the closed mold. In some cases, the preform may not be perfectly aligned, or may be moved off center by the action of the stretch rod acting inside the preform during stretching. Damage often occurs when the blow mold halves close onto the preform before the preform is even blown. Consequently, the blown article's body may not be aligned with its neck finish. The '664 reference forms an internal thread in the blown part and the blow nozzle must be unscrewed from the part before the mold can be opened. U.S. Pat. Nos. 4,818,212 and 4,036,573 also show typical stretch blow nozzle arrangements.
U.S. Pat. No. 5,137,443 discloses a holding device for holding the parison within the blow mold and first and second locking portions affixed to the holding device and blow mold portions, respectively, engagable with each other when the blow mold portions are in a closed position to align and clamp the holding device to the blow mold, whereby blowing force is transmitted to the engaged locking portions.
Accordingly, it is a principal object of the present invention to provide an apparatus for forming hollow plastic articles which includes a blow mold and holding means for holding a parison within the blow mold, wherein the apparatus includes means for conveniently and effectively aligning the blow mold portions.
It is a further object of the present invention to provide a versatile apparatus as aforesaid which permits the use of high pressure fluid without leakage and which permits accurate alignment reproducably over many operating cycles.
It is a still further object of the present invention to provide an apparatus as aforesaid which is suitable for use with different size parisons and which permits accurate positioning and clamping with little chance of parison damage.
Further objects and advantages of the present invention will appear hereinbelow.